In fluid control applications, the leakage associated with conventional spool valve elements is undesirable and in certain applications cannot be tolerated. In the latter circumstances, additional circuit valve elements must be employed to prevent such leakage in the spool's closed position to effect the control function desired. Even expensive and precise manufacturing methods such as "selective fit" technique cannot totally eliminate the normal leakage associated with valve spool elements which must slide within the spool bore in a close fit relationship.
On the other hand, two-way poppet elements have become popular because precise manufacture permits a seated metal to metal relationship to be obtained between the poppet element and the poppet seat which essentially prevents leakage. However, it is much more difficult to obtain precise control using normal poppet valve elements under modulating control conditions as compared to the very precise control under such conditions obtainable with spool type elements.
Given the fact that poppets work well in a system requiring only a simple open or closed condition and prevents leakage problems and spool valves offer precise control of modulating conditions but present a leakage problem, the prior art has failed to provide a valve assembly wherein only the desirable characteristics of both are combined in a single practical assembly wherein the valve element functions as a normal spool element for operative flow control functions and also includes the non-leakage attribute of a poppet element only for closed or holding positions.
Prior to the present invention, leakage problems related to spool valves were attacked by very expensive, precision manufacturing techniques and/or long lap distances between ports. However, this only tends to reduce, not eliminate, leakage inevitable between a spool movable in a bore. If control considerations permitted, a poppet valve would be employed, however, the instability and lack of precision control inherent under modulating control conditions often do not warrant use of a poppet valve.